Spectroscopy, such as rotational spectroscopy, is a powerful structural tool in physical chemistry. For example, the relationship between the molecular structure and the rotational transition frequencies can be used for structure determination of gas phase samples. Other effects in the rotational motion of molecules, such as centrifugal distortion, hyperfine spectral structure from quadrupolar nuclei, or frequency shifts caused by tunneling motion, can be used to provide further characterization of the molecular structure and low frequency vibrational motions. Microwave spectrometers, such as using a waveguide, generally limit the size range of molecules that can be interrogated by pure rotational spectroscopy, such as because of the need for sufficient vapor pressure in the waveguide cell.
A molecular beam, Fabry-Pérot, Fourier transform microwave (FTMW) spectrometer can be used to perform time-domain microwave spectroscopy to provide sensitive detection of the rotational free induction decay (FID), such as following polarization by a microwave pulse. Such time-domain spectroscopy can achieve high frequency resolution without power broadening or the line shape distortion associated with the waveguide-based approach discussed above. Pulsed molecular beam sources have expanded the range of molecular systems amenable to analysis by rotational spectroscopy. Generally, the Fabry-Pérot FTMW spectrometer is a narrowband spectrometer. For example, the use of a cavity with high quality factor generally limits the measurable frequency bandwidth to less than 1 megahertz (MHz) in generally-available spectrometer designs. The microwave cavity serves two functions: it decreases the power requirements for the microwave polarizing pulse and it enhances the amplitude of the FID emission signal. Often these spectrometers are called “broadband” to indicate that they operate over a wide frequency range, typically about 10 gigahertz (GHz). However, the process of acquiring a spectrum over the full operating range of the spectrometer is laborious. The spectrum scanning process generally involves a series of steps where the cavity is precisely tuned to resonance, a narrow frequency range is measured (e.g., 500 kilohertz (kHz)), and the cavity is moved to its next position in the frequency tuning series.
A spectrum spanning a frequency range of several GHz can be obtained, but the Fabry-Pérot spectrometer design leads to long spectrum acquisition times (e.g., many hours). A major contributor to the overall measurement time comes from the positioning of the cavity mirrors at each frequency step. The time-consuming spectral acquisition process poses difficulties for using FTMW spectroscopy in analytical chemistry applications, for optimizing source conditions for previously unknown species, or for performing rotational spectroscopy of excited vibrational or electronic states prepared by laser.